The present invention relates to digital transmission technology and particularly to transmission concepts particularly well suited for time-varying transmission channels as can be found in mobile radio and broadcasting.
Time interleaving and/or frequency interleaving combined with error-correcting codes (forward error correction, FEC) belong to a basic principle in transmission technology, as shown in FIG. 6.
An information word consisting of information bits here is input in an FEC encoder establishing a codeword from this information word, i.e. a vector of code symbols or code bits. These codewords and/or blocks formed therefrom are passed to the interleaver. It changes the order of the symbols and passes the symbols thus mixed onto the transmission channel. The re-sorting of the symbols may take place in the time axis (“time interleaving”) and/or in the frequency axis (“frequency interleaving”).
The use of an interleaver makes sense if the transmission channel is not static, i.e. if its properties change with time and/or frequency. Thus, the signal power arriving in the receiver may vary strongly in a receiver being moved. Thereby, some code symbols are faulty with higher probability (e.g. by superposed thermal noise) than others.
Depending on the movement of transmitter, receiver and/or objects along the transmission path and depending on the nature of the surroundings of transmitter, receiver and transmission path, the channel properties may change more or less quickly. A measure of the temporal constancy of the transmission channel is the coherence time: the channel does not change significantly in this time.
The probability of a transmission error usually is estimated from the channel state. The channel state describes the quality of the reception signal (e.g. the momentary ratio of signal strength to noise). It is the aim of an interleaver to distribute the information in time (and often also in frequency) so that, with time-varying channel properties, the ratio of “good” (small probability of a transmission error) to “bad” (high probability of a transmission error) symbols becomes approximately temporally constant on average behind the de-interleaver, which reverses the interleaver on the transmission side. In the case of a quickly changing channel property (e.g. high vehicle speeds), usually a relatively short interleaver is sufficient. With slowly time-varying channel properties, a correspondingly greater interleaver length should be chosen.
The change in the channel properties may result from various effects.
In the case of multi-path propagation, the relative phase location of the signal proportions determines whether the signal proportions superimpose constructively or destructively. Even a change in position by a fraction of the wavelength of the carrier signal here leads to other phase locations. The channel properties may change correspondingly quickly. This is referred to as “fast fading”.
The signal properties do, however, also strongly depend on the surroundings. Thus, e.g. walls attenuate the signal. Correspondingly, the signal quality within a house usually is worse than outside. The change in the signal properties correlated with the surroundings changes slowly as compared with the fast fading. Correspondingly, this is referred to as “slow fading”.
Usually, only the properties of the fast fading are considered in the interleaver design. Since memory costs become less and less, however, increasingly very long interleavers now also become interesting. In this case, the properties of the slow fading have to be considered to an increasing extent in the interleaver design.
The following may be mentioned as examples for slow fading:
Mobile reception of satellite signals. For a moving car, the reception scenario constantly changes corresponding to the surroundings. For each reception scenario, three reception states may be defined.
There is a line-of-sight link to the satellite (e.g. open road). This is referred to as “line-of-sight state” (LOS)
The signals are attenuated (e.g. by trees). This state often is referred to as “shadow state”
The signal is attenuated so heavily that it is no longer useful. This is often referred to as “blockage state”.
Transmission in cellular networks with transmitters of relatively low transmission power.
In cellular networks area coverage is achieved by many transmitters. For this kind of networks, it has to be reckoned with the fact that the reception conditions change relatively quickly. Since the transmitter distance is small, the relative distance of the receiver may change quickly. In this case, the signal properties in long interleavers may change strongly already within the interleaver length.
In the receiver, the exchange of code symbols (=interleaving) performed in the transmitter is reversed again (=de-interleaving). This leads to the fact that burst errors occurring in the transmission are distributed as individual errors to the entire data block behind the de-interleaver and my thus be corrected more easily by the FEC decoder.
The following interleaver types are to be distinguished:
convolution interleaver
block interleaver
Convolution interleavers deal with “inter-block interleaving”, i.e., blocks are “blurred” temporally such that blocks being in succession before the interleaver are intertwined behind the interleaver. Here, a block is formed of one or more codewords. The interleaver length does not depend on the block size, but on the width of the blurring.
In an exemplary convolution interleaver, a block of FEC code symbols is divided into e.g. four partial blocks of unequal size by the interleaver and intertwined with the upstream and/or downstream blocks.
Convolution interleavers are characterized in that the output of the FEC encoder is divided into various partial data streams via a de-plexer. The principle is illustrated in FIG. 7. Here, the data stream usually is distributed to the partial data streams in bit-wise manner or in groups of bits (“symbols”). Each partial data stream then is delayed via delay lines (e.g. implemented via FIFOs).
For synchronization of the convolution de-interleaver in the receiver, only the de-multiplexer needs to be synchronized.
The length of the delay lines may be regularly stepped. Any arrangements may be chosen, however, so that successive symbols lie as far apart as possible and the channel properties therefore are uncorrelated.
Block interleavers deal with “intra-block interleaving”, i.e. the processing takes place in block-wise manner, with one block consisting of one or more codewords. The block size here defines the interleaver length. Here, systematic FEC codes frequently are employed; the data block here contains useful information (=the information to be transmitted) and additional redundancy, in order to be able to correct transmission errors.
Various types of block interleavers are known.
It is the basic principle of a block interleaver that the elements of a data vector or matrix are permuted, i.e. exchanged.
The variant of the block being taken for a matrix is best known. One row here forms e.g. one codeword (e.g. a Reed-Solomon codeword). The information then is copied into the matrix row by row and read out column by column in the transmitter/interleaver. As an example, the method from the ETSI Standard EN301192, which is illustrated in FIG. 8, is to be mentioned here.
FIG. 9 shows the arrangement of the useful data (“application data”). Reading out and/or transmitting then takes place in datagrams, with FIG. 9 further showing a matrix arrangement in rows, wherein the matrix has a number of rows equal to “no of rows”. Furthermore, as an example, there is a number of columns extending from a number 0 to a number 190. In order to fill the matrix, so-called padding bytes continuing (cont.) up to the last padding bytes are added after the last datagram.
The interleaver properties may, among other things, be characterized by the following parameters:
End-to-End Delay:
This parameter defines the time interval between the time instant when the symbol is available at the input of the interleaver until the time instant when this symbol is available at the output of the de-interleaver.
(Receiver) Access Time
Time interval between the time instant when the first symbol is available at the input of the de-interleaver and the time instant when the codeword is available and decodable at the input of the FEC decoder, which means at the output of the de-interleaver. According to the invention, one only needs to wait until a sufficiently large part of the codeword is available at the output of the de-interleaver, and not the complete time of the end-to-end delay, as long as the received packets have a sufficient signal-to-noise ratio. This parameter determines the time between switching on the receiver or switching to another program and the availability of the signal (e.g. audio or video signal) for the user e.g. in a broadcast receiver. Decoding of e.g. a video signal under some circumstances may mean further delay, which should not go into the access time, however. In this respect, it is to be noted that an audio or video decoder could generate further delay also having an effect on services not being time-interleaved.
Memory Requirement
The memory requirement is determined by the interleaver length and the interleaver type as well as the chosen representation of signals in the transmitter or receiver.
The above-described interleaver concepts are characterized by good scrambling both within a codeword or block and beyond codeword boundaries in temporal respect. As illustrated in FIG. 7, a change in the order of the individual symbols in a codeword serially entering the input-side de-multiplexer is achieved by the delay elements in the outer interleaver. With respect to the transmission of these data, this does not have to be temporal scrambling here, however, but frequency scrambling may also be achieved therewith. Frequency scrambling is achieved, for example, if the data stream output from the multiplexer at the right-side end of the outer interleaver is serial-parallel converted and associated with a set of e.g. 1024 carriers in an OFDM symbol, so that two bits of the output-side data stream are associated with a carrier if QPSK mapping is used, for example, so that an OFDM occupation accommodates 2048 bits in the order as generated by the outer interleaver. Naturally, this means that bits and/or FEC symbols are arranged on other carriers as they would have been arranged if the outer interleaver had not been present, due to the delay elements in the outer interleaver.
A convolution interleaver or interleaving interleaver with delays thus works either as time interleaver or as frequency interleaver or both as time and frequency interleaver, depending on the subsequent implementation.
It is disadvantageous in the interleaver structure shown in FIG. 7 that there are high expenditure and high memory requirements both on transmitter side and on receiver side. This disadvantage becomes increasingly grave, the bigger the codewords become, i.e. the more bits are input as a block into an FEC encoder, and the more bits are output as a block from the FEC encoder, as shown in FIG. 6, for example. FEC encoders have code rates smaller than 1. A code rate of ⅓, for example, means that the number of bits in a codeword output from the FEC encoder is three times the number of bits in an input block or information word input into the FEC encoder, as outlined in FIG. 6. The interleaver now is to perform as good a temporal and frequency scrambling as possible, so that a multiplexer control, and/or generally speaking “processing” of its own, is needed for every bit and/or for every byte (depending on the coding scheme of the FEC).
This directly entails that a corresponding de-interleaver control is needed on the receiver side as well. Furthermore, quality information, such as a value for an achieved signal/noise ratio, for a bit error probability or a probability for the value of the bit and/or byte, has to be generated for decoding for each bit and/or for each symbol, wherein such probabilities are employed especially in so-called soft decoders. While this is not that critical in relatively small codewords yet, the problem increases, the longer the codewords become. For reduced transmitter complexity and particularly for reduced receiver complexity, which is particularly critical for broadcasting applications, since the receivers are mass products and have to be offered cheaply, this means that actually a small codeword length is desirable. On the other hand, a greater codeword length provides better advantages with slowly time-varying channel properties, since a codeword can be “distributed” over a longer period of time and/or a greater frequency range.